1. Field of the Invention
This invention relates to the preparation of polyamines and is more particularly concerned with a process for the condensation of aniline and formaldehyde to yield polyamine mixtures containing a high proportion of diaminodiphenylmethane and low oligomeric polyamine content.
2. Description of the Prior Art
The preparation of mixtures of methylene bridged polyphenyl polyamines containing diaminodiphenylmethanes by condensation of aniline and formaldehye under aqueous conditions in the presence of mineral acids, particularly hydrochloric acid, has been widely described. Illustrative of such processes are those shown in U.S. Pat. Nos. 2,638,730; 2,950,263; 3,260,751; 3,277,173; 3,297,759; and 3,476,806. Such processes, in one form or another, are widely used commercially to prepare the polyamines in question, which polyamines are employed as intermediates in the preparation of the corresponding isocyanates, i.e. diisocyanatodiphenylmethane and mixtures of polymethylene polyphenyl polyisocyanates. The isocyanates are employed in the preparation of a variety of polyurethanes, polyisocyanurates and other polymers (both cellular and non-cellular) which can be derived from polyisocyanates.
However, the operation of processes catalyzed by aqueous mineral acid of the type described above necessarily gives rise to serious corrosion problems, involving constant repair and maintenance requirements, and increasing overhead costs in the operation of the manufacturing plants in which such processes are utilized.
Very little attention has been devoted hitherto to utilizing catalysts of a relatively non-corrosive nature. U.S. Pat. No. 3,362,979 describes the use of siliceous catalysts at elevated temperatures (125.degree. to 300.degree. C. preferred). The aniline and aqueous formaldehyde are brought together in the presence of the catalyst at a temperature in the above range and the water introduced with the formaldehyde, as well as the water of condensation eliminated in the reaction, is removed from the reaction mixture on a continuous basis. Such a procedure does not lend itself readily to operation on a continuous basis on a commercial scale. Further, the diamine content in the product in many instances is relatively low and said diamine contains excessively high proportions of 2,4'-isomer.
British patent specification No. 1,207,377 describes a very similar process, i.e. reaction of aromatic amine and formaldehyde at elevated temperatures with continuous removal of water, but using a catalyst comprising a benzene sulfonic acid supported on an inert material. No details are given of the results of application of the process to the condensation of aniline and formaldehyde, all the specific examples being devoted to the condensation of either mono- or dichloroaniline with formaldehyde.
French Patent Specification 1,448,359 shows a similar process using, for example, a bentonite clay with removal of water from a mixture of aniline and formaldehyde while the latter is refluxed in the presence of the clay catalyst.
The above procedures have, in our hands, been found to give rise to rapid deactivation of the catalyst with consequent need to reactivate the catalyst after each run. The procedures are not readily adapted to continuous operation on the commercial scale. Further, the yield of diamine in the polyamine product is relatively low and the proportion of 2,4'-isomer present in the diamine is relatively high and certainly higher than is desirable for reasons which will be explained hereafter.
We have now found that the condensation of aniline with formaldehyde, and the conversion of the initial condensation product to the desired polyamines having high diamine content, can be effected by heterogeneous catalysis using improved procedures which are free from the problems discussed above. Not only do the procedures described below have the advantage of being corrosion free, but they also have been found to yield results which are unexpected and highly useful.